This was the last use of `CustomDataAttributes`. It's not very useful
nowadays because we have a lower level (`CustomData`) and higher level
(`AttributeAccessor`) API.
As part of this I removed the ability to `reserve` instances which was
useful when adding instances one by one. Generally, such code should
eventually be rewritten to handle more instances at once, instead of
handling them one by one. This will lead to better performance and
is more in line with how other geometry types (like mesh) are handled.
This also adds the ability to move and assign `Instances` just like we can
move and assign `CurvesGeometry`.
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